As used herein, a "charged particle beam" is a beam of charged particles such as electrons or ions. For simplicity, the following discussion is in the context of an electron beam; however, it will be understood that the principles of the invention can be applied with equal facility to other types of charged particle beams.
In conventional electron-beam microlithography apparatus, an electron beam is produced by an electron gun. The electron beam passes through an "illumination-optical system" to illuminate a portion of a patterned reticle. The reticle defines the pattern (e.g., a layer of an integrated circuit) to be transferred to a sensitized substrate (e.g., semiconductor wafer). The beam between the electron gun and the reticle is termed an "illumination beam." After passing through the illuminated portion of the reticle, the beam (now termed an "imaging beam" or "patterned beam") passes through a "projection-optical system" to form a corresponding image on the surface of the substrate. The substrate surface is "sensitized" by a previously applied layer of a suitable resist that is responsive in an image-forming way to exposure to charged particles of the imaging beam. For exposure, the dosage of charged particles impinging on the surface of the substrate can be increased or decreased by increasing or decreasing, respectively, the beam current.
In electron-beam microlithography systems, if the beam current is increased (e.g., in an effort to increase throughput), the electron density in the beam is correspondingly increased, which results in correspondingly increased Coulomb repulsion between electrons in the beam. Such Coulomb repulsion, also termed a "space-charge effect," causes the beam to spread out, which causes blurring of the image transferred by the beam.
Certain types of electron-beam microlithography apparatus are termed "critical illumination" systems in which an enlarged image of a crossover produced by the electron gun is formed on a downstream beam-shaping aperture in the illumination-optical system. In conventional illumination-optical systems intended for critical illumination, the transverse intensity profile of the electron beam (i.e., intensity profile of the beam in a plane perpendicular to the optical axis of the illumination-optical system or projection-optical system) exhibits a Gaussian distribution. In the Gaussian distribution of the beam in conventional systems, the center of the beam has the highest intensity, with intensity falling off rapidly with increasing distance from the beam center. For example, the portion of the beam at the center of the distribution where the beam intensity is flat to within .+-.1% has a diameter of 1/8 or less of the total beam diameter. As a result, with critical illumination, an exposure area that is the same as that obtainable with Kohler illumination cannot be obtained without increasing the current supplied to the electron gun. But, as noted above, increasing the gun current increases the beam current, which increases space-charge effects. (In a Kohler illumination system, the beam diverging from a crossover is incident to a field-limiting aperture, and the crossover is imaged in the entrance pupil of the projection-optical system.)
A beam having a Gaussian intensity distribution with a center peak intensity is termed a "solid" beam. In a solid beam, space-charge effects are a major problem.
"Hollow" beams are known. According to the reference Ura, Katsumi, Electron Optics, Kyoritsu, 1979, a hollow beam exhibits less beam spreading due to space-charge effects. A hollow electron beam can be generated, for example, using an electron gun having a frusto-conical cathode, wherein the conical surface of such a cathode is the electron-emitting surface. A gun crossover is typically located just downstream of the cathode of such an electron gun. The current density at the gun crossover of an electron gun with a frusto-conical cathode exhibits a Gaussian distribution. Certain of such electron guns also have an associated gun lens. So long as any aberrations generated by the gun lens are small, the angular distribution of the electron beam emerging from the crossover will be characteristic of a hollow beam. If the gun lens or any other portion of the electron gun exhibits excessive aberration, however, the edges of the beam-intensity distribution become blurred, producing a beam that is no longer clearly hollow. Such aberrations are extremely difficult to correct or control at the gun. Even though a blurred hollow beam can be shaped to some extent by passing the beam through an annular aperture, this remedy alone is inadequate for forming the desired quality of hollow beam. In addition, attempting to "correct" a blurred beam using an annular aperture in such a manner results in blocking a large proportion of the beam from propagating downstream of the annular aperture to, e.g., the reticle. Consequently, a very large beam current is required which further aggravates space-charge effects and causes an excessive temperature rise of the annular aperture.
With certain conventional electron-beam microlithography systems, the reticle pattern is divided into multiple exposure units (e.g., stripes, subfields, or the like, wherein an "exposure unit" is the area on the reticle that is illuminated, and thus exposed, by the beam at any given instant of time). Each exposure unit defines a respective portion of the overall pattern defined by the reticle. The exposure units typically exhibit differing feature densities from one exposure unit to another and can exhibit substantial differences in feature density within individual exposure units. Differences in feature density result in corresponding differences in downstream beam current. As a result, under such conditions, points of best focus of the beam at the substrate are not in the same plane.